GOLD is the most fixed of all the metals; and it hath been generally believed, till very lately, that it was absolutely fixed in every degree of fire which could be excited by human art. Some chemists indeed pretended to have evaporated, calcined, and even vitrified gold, by exposing it to the focus of a good burning-glass; but as their experiments did not appear to have all the authenticity that was necessary, the opinion fell into discredit; and this the more readily that gold is found to resist the most violent furnace fires without any loss. Mr Macquer, however, in the last additions to his Chemical Dictionary, hath found, that this metal, when exposed to the heat of a good burning-glass, loses in weight considerably, and that even in a very short time. He held some very fine gold in the focus of a large burning-glass, at several times, at half an hour each time, sometimes in a hollow piece of charcoal, or in vessels made of earthen ware and porcelain; and whenever the air was very clear, and the sun bright, a very sensible smoke arose to the height of three or four inches. To know the nature of this smoke, a cold silver-plate was exposed to it; by which means, some of the vapour appeared on the silver like a tarnish a little less white, but not sensibly yellow. But when this part of the silver was rubbed with a burnisher, it appeared so evidently to be gilded, that none could doubt the evaporation of the metal. At this experiment several members of the Academy of Sciences were present.
The experiments with regard to the calcination and vitrification of gold were also tried by Mr Macquer, in presence of Messrs Montigny, Cadet, Lavoisier, and Brisson. The glass employed was that of Tschirnhausen, the same which had formerly been employed by Mr Homberg. Gold of 24 carats fine was exposed to the focus of this glass several times, by half an hour at a time, on vessels made of a very refractory porcelain earth.—When the gold was melted, which
Gold. which happened in the space of a few seconds, it took a spherical form as nearly as its gravity would permit, and had nearly the consistence of quicksilver. The melted mass soon acquired a rotatory motion round its axis, sometimes in one direction, and sometimes in another, according to the direction of the focus upon the metal; (which, by-the-bye, is a very curious appearance, and seems difficult to be accounted for). A smoke arose as in the former experiments. On the surface of the melted gold, spots evidently vitrified were formed, which separated from the metal. These afterwards united into a dark violet-coloured mass, of a greater curvature than the melted gold, in which they were found to be set, as jewels in a ring; and produced nearly the same appearance that the transparent cornea has on the globe of the eye; that is, as the segment of a small sphere joined to the surface of a large one. On a mass of liquid gold, of about half an inch diameter, he obtained, after four hours calcination, a button of this violet glass, whose diameter exceeded two lines; and this glass was observed gradually to increase as the gold diminished. Yet, though this very much resembles the vitrification of the gold, Mr Macquer doth not take upon him to assert that it certainly is so; seeing it may arise from the vessels on which the gold is supported, and which were coloured purple to a considerable distance, and besides were sprinkled with many small particles of gold undecomposed, easily distinguishable by a microscope; and in the violet glass also, many similar particles were observable. To determine the matter accurately, the experiment ought to be continued till all the particles of the metal be certainly either vitrified or evaporated.—Though this experiment, however, is hitherto equivocal when the heat of burning-glasses is made use of, it certainly succeeds with a strong flash of electric fire. See ELECTRICITY, n° 113.
Method of Recovering Gold from Gilt Works. The solubility of gold and the indissolubility of silver in aqua regia affords a principle on which gold may be separated from the surface of silver; and, on this foundation, different processes have been contrived, of which the two following appear to be the best.—Some powdered sal ammoniac, moistened with aqua fortis into the consistence of a paste, is spread upon the gilt silver, and the piece heated, till the matter smokes, and becomes nearly dry: being then thrown into water, it is rubbed with a scratch brush, composed of fine brass-wire bound together, by which the gold easily comes off. The other way is, by putting the gilt silver into common aqua regia, kept so hot as nearly to boil, and turning the metal frequently till it becomes all over black: it is then to be washed with a little water, and rubbed with the scratch brush, to get off what gold the aqua regia may have left. This last method appears preferable to the other; as the same aqua regia may be made to serve repeatedly till it becomes saturated with the gold, after which the gold may be recovered pure by precipitation with solution of vitriol, as directed under the article METALLURGY.
For separating gold from gilt copper, some direct a solution of borax to be applied on the gilt parts, but nowhere else, with a pencil, and a little powdered sulphur to be sprinkled on the places thus moistened; the principal use of the solution of borax seems
to be to make the sulphur adhere; the piece being then made red hot, and quenched in water, the gold is said to be so far loosened, as to be wiped off with a brush. Others mix the sulphur with nitre and tartar, and form the mixture with vinegar into a paste, which is spread upon the gilt parts. Gold.
Schlutter recommends mechanical means, as being generally the least expensive, for separating gold from the surface both of silver and copper. If the gilt vessel is round, the gold is conveniently got off by turning it in a lathe, and applying a proper tool, a skin being placed underneath for receiving the shavings: he says it is easy to collect into two ounces of shavings, all the gold of a gilt vessel weighing thrice as many pounds. Where the figure of the piece does not admit of this method, it is to be properly fixed, and scrapers applied, of different kinds, according to its size and figure; some large, and furnished with two handles, one at each end; others small and narrow, for penetrating into depressed parts. If the gold cannot be got off by either of these ways, the file must be had recourse to, which takes off more of the metal underneath than the turning tool or the scraper, particularly than the former. The gold scrapings, or filings, may be purified from the silver or copper they contain, by the methods described under the article METALLURGY.
The editors of the Encyclopédie give a method of recovering the gold from wood that has been gilt on a water-size: this account is extracted from a memoir on the same subject, presented to the Academy of Sciences by M. de Montamy. The gilt wood is steeped for a quarter of an hour, in a quantity of water, sufficient to cover it, made very hot: the size being thus softened, the wood is taken out, and scrubbed, piece by piece, in a little warm water, with short stiff bristle brushes of different sizes, some small for penetrating into the carvings, and others large for the greater dispatch in flat pieces. The whole mixture of water, size, gold, &c. is to be boiled to dryness, the dry matter made red hot in a crucible to burn off the size, and the remainder ground with mercury, either in a mortar, or, where the quantity is large, in a mill.